Imagine you are an early human, a hunter gatherer out hunting in your east African home. You’re relying on instinct to keep you alive. There’s a rustle in a bush a few hundred yards behind you, there’s something hiding there: it might be something you could eat, but it might be something bigger that could eat you. Could it be a lion? You weren’t even looking in that direction, you were concentrating on something in front of you, but System 1 drew your attention to it with a message containing a strong emotion – fear. You run for your life, hungry but still alive. The very purpose of emotions is thought to be that they are the mechanism by which System 1 gets System 2’s attention and influences its decisions.

As well as protecting themselves our genes evolved to propagate themselves. Our unconscious attributes inanimate objects with emotions and human characteristics so that we can use them to signal our characteristics to possible mates.

Everything we sense is assessed as to whether it is significant.

There is far too much going on around us for us to consciously - System 2 - evaluate.

System 1 does it for us quickly and effortlessly, we are not aware of it doing it. It picks up clues from a stimulus; it is particularly good at noticing change and anything with strong emotions attached. These clues generate associations with memories - and the emotions associated with them - stored in our brain and compares them to the stimulus.

One thing that the higher system, System 2, is good at is balancing probabilities and possibilities but this also illustrates a key weakness of System 2: it is not decisive. Most of us have experienced how the more we think about a decision, the more we go round in circles and fail to reach a conclusion. In the event that it was a lion behind that bush, this is not decisive or fast enough. By the time we reach a conclusion, we'd be lunch. By contrast, the more primitive System 1 is decisive and fast. It is this system we use when we make decisions. And it works in a different way to System 2: it doesn’t consider possibilities and probabilities and is not rational.

It is possible for our conscious System 2 to override our unconscious System 1. But it’s rare for us to do so. In a moment we will look at the relationship between the two systems but most of the time we go along with the message from System 1. Modifying the message is hard work; it requires effort, is slow and quite difficult to do. We do not have the time or inclination to alter most of our decisions and even when we do System 2 is fundamentally indecisive.

System 1 makes most of our decisions. System 2 may modify that decision but rarely does. If we want to understand our decisions we need to understand how System 1 works and how the two systems relate to each other.